The present invention relates to a method and apparatus for preparing gypsum products (i.e., products comprising calcium sulfate dihydrate) from starting materials comprising calcined gypsum (i.e., calcium sulfate hemihydrate or anhydrite) and water. More particularly, the present invention relates to an improved apparatus for use in conjunction with the slurry mixer typically used in supplying agitated gypsum slurry to a wallboard production line. The basic technology of gypsum wallboard manufacture is disclosed in U.S. Pat. Nos. 1,500,452; 2,207,339; and 4,009,062 all of which are incorporated by reference herein. The present apparatus provides an improved mixing chamber which enhances the uniform mixing of foam into the gypsum slurry.
It is well known to produce gypsum products by uniformly dispersing calcined gypsum in water to form a slurry and then casting the slurry into a desired shaped mold or onto a surface and allowing the slurry to set to form hardened gypsum by reaction of the calcined gypsum (calcium sulfate hemihydrite or anhydrite) with the water to form hydrated gypsum (calcium sulfate dihydrate). It is also well known to produce a lightweight gypsum product by uniformly mixing an aqueous foam into the slurry to produce air bubbles. This will result in a uniform distribution of voids in the set gypsum product if the bubbles do not escape from the slurry before the hardened gypsum forms. The voids lower the density of the final product, which is often referred to as xe2x80x9cfoamed gypsumxe2x80x9d.
Prior apparatus and methods for addressing some of the operational problems associated with the production of foamed gypsum are disclosed in commonly-assigned U.S. Pat. Nos. 5,638,635 and 5,643,510, which are incorporated by reference. The present invention relates generally to the use of foamed gypsum in the production of gypsum wallboard.
A gypsum wallboard mixer typically includes a housing defining a mixing chamber with inlets for receiving calcined gypsum and water, among other additives well known in the art. The mixer includes an impeller or other type of agitator for agitating the contents to be mixed into a mixture or slurry. Such mixers typically have a rectangular discharge gate or slot with a cutoff block or door. The discharge gate controls the flow of slurry from the mixer, and is difficult to adjust to change slurry flow when product requirements change, such as when thicker or thinner wallboard is desired.
Foam and/or other additives are generally added through a foam slot on the outer side of the cut-off block and about 2-4 inches before the gate""s connection to a vertical canister and a donut or restrictor ring. In the production of gypsum wallboard, a balance must be maintained in that the foam is uniformly mixed into the slurry, but not broken down from excessive agitation. Within the canister, which is approximately 5-7 inches in diameter, the material spins downward. The canister is employed to reduce the flow pressure of the slurry discharged from the mixer, which typically operates in the range of 270-300 rpm. One drawback of such canisters is that a centrifugal force set up within the canister causes separation between the materials in the mix due to density differences.
More specifically, recent studies have shown that a vortex is created as the mixture flows in the canister, which also creates an empty air space in the canister. Such an air space is typically representative of unwanted buildup of material in the canister, which then more easily sets and causes clogging of the apparatus. Clogged mixing equipment causes costly downtime for repairs.
Another disadvantage of the vortex created in the canister is that higher density slurry components become separated from the relatively lower density foam. Due to centrifugal force, the slurry is pushed to the sides of the canister and the foam tends to stay in the middle. Thus, rather than providing a site for the uniform mixing of the foam and the slurry, the canister appears to be doing the opposite.
From the canister and donut ring, the material typically flows into a flexible, generally horizontal distribution boot, from where it is dispensed onto the wallboard paper web traveling with the flow of material. In some applications, the slurry is dispensed upon a previously deposited layer of relatively denser gypsum slurry. If the pressure of the dispensed slurry is too high, the previously deposited layer is disturbed, resulting in a condition known as xe2x80x9cwashoutxe2x80x9d.
Typically, a second facing is then applied on top of the slurry to constitute the second face of the gypsum board. Next, the sandwich passes through a forming station which determines the width and thickness of the gypsum board. This process is carried out in a substantially continuous operation, and the gypsum slurry begins to set immediately after the board is formed. Subsequently, the board is dried, cut and bundled into commercially acceptable lengths. Since the uniform mixing of foam within the slurry is a desired result in wallboard manufacture, in view of the drawbacks of the canister, the only places where the foam can mix with the slurry are the mixer gate and the flexible rubber boot.
Thus, it is an object of the present invention to provide an improved gypsum slurry mixing apparatus which does not require a canister.
Another object of the present invention of the present invention is to provide an improved gypsum slurry mixing apparatus and method which promotes uniform mixing of foam into the slurry.
Still another object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which causes a reduction in slurry flow pressure prior to discharge of the mixture upon the wallboard paper and at the same time promotes even mixing of foam into the slurry.
A further object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which reduces maintenance of mixing and dispensing equipment due to premature setting of the slurry.
A still further object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which increases slurry/foam mixing time while not requiring additional length of the board production line.
Yet another object of the present invention is to provide an improved gypsum slurry mixing apparatus and method which provides an easily accessible mechanism for changing the volume of slurry emitted from the mixer.
Accordingly, the above-listed objects are met or exceeded by the present apparatus and method for controlling the output of a slurry mixer including the features of eliminating the canister and its undesirable vortex and replacing it with an extended mixing and dispensing apparatus. The extended mixing and dispensing apparatus gate preferably includes an elongate, preferably flexible conduit which provides additional space for uniform mixing of slurry and additives such as foam, retarders, dispersants and accelerators. By providing a flexible mixing chamber in the conduit, unwanted premature setting of the gypsum is prevented so that occurrences of mixer or conduit lumps and/or paper breaks are reduced. Also, coiling of the chamber is available for applications in which there is limited space in the board line between the mixer outlet and the board forming plate. In the latter application, an extended length mixing chamber is provided which is applicable with board lines having limited space.
Another feature of the present invention is the inclusion of an adjustable pinch or concentric valve located in the preferably flexible conduit for regulating slurry flow to create backpressure in the mixer, which further enhances the uniform mixing action of the foam and slurry. This backpressure also keeps the mixer filled with slurry and thus prevents unwanted premature setting and clogging. When the valve is a concentric valve, it exerts a uniform pressure around the entire circumference of the conduit, which also reduces premature setting and clogging. Regardless of the type of valve, the use of a valve in association with the flexible conduit provides a convenient way to change the volume of dispensed slurry and thus adjust the thickness of produced wallboard as a result.
Yet another feature is the provision of a shock-absorbing device as a part of the conduit to reduce the force or pressure of the dispensed slurry upon the receiving web of substrate or previously dispensed slurry. In the preferred embodiment, the shock-absorbing device takes the form of a generally xe2x80x9cSxe2x80x9d or xe2x80x9cCxe2x80x9d-shaped double bend formation.
More specifically, an apparatus for delivering an additive-enhanced slurry for use in making gypsum wallboard, and configured for connection to a centrifugal mixer with a tangential discharge outlet, includes a conduit having a main inlet in slurry receiving communication with the mixer outlet and extending to a spout for discharging the slurry proximate a wallboard forming area. At least one volume restrictor is associated with the conduit for creating backpressure between the volume restrictor and the mixer outlet for keeping the mixer full, and at least one pressure reducer is associated with the discharge spout and configured for reducing the pressure of the slurry dispensed from the discharge spout. The apparatus is configured for maintaining a generally laminar flow from the mixer outlet to the discharge spout.
In the preferred embodiment, the dispensing apparatus is provided with at least one preferably adjustable valve for creating backpressure in the conduit and mixer, for controlling the flow of slurry from the spout and for reducing buildup of slurry in the apparatus and the mixer. It is preferred that the apparatus, including the preferably flexible conduit portion, be connected directly to the mixer outlet so that generally laminar flow is preserved from the mixer to the dispensing point on the wallboard production line.
In still another embodiment, a method for providing an evenly mixed additive enhanced slurry to a web is provided, including inserting calcined gypsum and water into a mixing chamber of a mixer through one or more inlets of the mixing chamber, agitating the contents of the mixing chamber to form an aqueous dispersion of the calcined gypsum, emitting the agitated contents from an outlet of the mixer, passing the agitated contents into a dispensing apparatus, introducing an aqueous foam into the mixture, creating a backpressure on the mixture in the apparatus by reducing the volume of mixture being emitted from a conduit of the apparatus, the back-pressure being created by constricting the conduit, and controlling the pressure of slurry and foam dispensed from the spout.